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        <div>
            <div><h3 class="title">
                <a name="asio.reference.async_compose"></a><a class="link" href="async_compose.html"
                                                              title="async_compose">async_compose</a>
            </h3></div>
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    <p>
        <a class="indexterm" name="asio.indexterm.async_compose"></a>
    </p>
    <p>
        Launch an asynchronous operation with a stateful implementation.
    </p>
    <pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span>
    <span class="keyword">typename</span> <span class="identifier">CompletionToken</span><span class="special">,</span>
    <span class="keyword">typename</span> <span class="identifier">Signature</span><span class="special">,</span>
    <span class="keyword">typename</span> <span class="identifier">Implementation</span><span class="special">,</span>
    <span class="keyword">typename</span><span class="special">...</span> <span
                class="identifier">IoObjectsOrExecutors</span><span class="special">&gt;</span>
<a class="link"
   href="asynchronous_operations.html#asio.reference.asynchronous_operations.automatic_deduction_of_initiating_function_return_type"><span
        class="emphasis"><em>DEDUCED</em></span></a> <span class="identifier">async_compose</span><span class="special">(</span>
    <span class="identifier">Implementation</span> <span class="special">&amp;&amp;</span> <span class="identifier">implementation</span><span
                class="special">,</span>
    <span class="identifier">CompletionToken</span> <span class="special">&amp;</span> <span
                class="identifier">token</span><span class="special">,</span>
    <span class="identifier">IoObjectsOrExecutors</span> <span class="special">&amp;&amp;...</span> <span
                class="identifier">io_objects_or_executors</span><span class="special">);</span>
</pre>
    <p>
        The async_compose function simplifies the implementation of composed asynchronous
        operations automatically wrapping a stateful function object with a conforming
        intermediate completion handler.
    </p>
    <h5>
        <a name="asio.reference.async_compose.h0"></a>
        <span><a name="asio.reference.async_compose.parameters"></a></span><a class="link"
                                                                              href="async_compose.html#asio.reference.async_compose.parameters">Parameters</a>
    </h5>
    <div class="variablelist">
        <p class="title"><b></b></p>
        <dl>
            <dt><span class="term">implementation</span></dt>
            <dd><p>
                A function object that contains the implementation of the composed
                asynchronous operation. The first argument to the function object is
                a non-const reference to the enclosing intermediate completion handler.
                The remaining arguments are any arguments that originate from the completion
                handlers of any asynchronous operations performed by the implementation.
            </p></dd>
            <dt><span class="term">token</span></dt>
            <dd><p>
                The completion token.
            </p></dd>
            <dt><span class="term">io_objects_or_executors</span></dt>
            <dd><p>
                Zero or more I/O objects or I/O executors for which outstanding work
                must be maintained.
            </p></dd>
        </dl>
    </div>
    <h5>
        <a name="asio.reference.async_compose.h1"></a>
        <span><a name="asio.reference.async_compose.example_"></a></span><a class="link"
                                                                            href="async_compose.html#asio.reference.async_compose.example_">Example:</a>
    </h5>
    <pre class="programlisting"><span class="keyword">struct</span> <span
            class="identifier">async_echo_implementation</span>
<span class="special">{</span>
  <span class="identifier">tcp</span><span class="special">::</span><span class="identifier">socket</span><span
                class="special">&amp;</span> <span class="identifier">socket_</span><span class="special">;</span>
  <span class="identifier">asio</span><span class="special">::</span><span
                class="identifier">mutable_buffer</span> <span class="identifier">buffer_</span><span
                class="special">;</span>
  <span class="keyword">enum</span> <span class="special">{</span> <span class="identifier">starting</span><span
                class="special">,</span> <span class="identifier">reading</span><span class="special">,</span> <span
                class="identifier">writing</span> <span class="special">}</span> <span
                class="identifier">state_</span><span class="special">;</span>

  <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span
                class="identifier">Self</span><span class="special">&gt;</span>
  <span class="keyword">void</span> <span class="keyword">operator</span><span class="special">()(</span><span
                class="identifier">Self</span><span class="special">&amp;</span> <span
                class="identifier">self</span><span class="special">,</span>
      <span class="identifier">asio</span><span class="special">::</span><span
                class="identifier">error_code</span> <span class="identifier">error</span> <span
                class="special">=</span> <span class="special">{},</span>
      <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span
                class="identifier">n</span> <span class="special">=</span> <span class="number">0</span><span
                class="special">)</span>
  <span class="special">{</span>
    <span class="keyword">switch</span> <span class="special">(</span><span class="identifier">state_</span><span
                class="special">)</span>
    <span class="special">{</span>
    <span class="keyword">case</span> <span class="identifier">starting</span><span class="special">:</span>
      <span class="identifier">state_</span> <span class="special">=</span> <span class="identifier">reading</span><span
                class="special">;</span>
      <span class="identifier">socket_</span><span class="special">.</span><span
                class="identifier">async_read_some</span><span class="special">(</span>
          <span class="identifier">buffer_</span><span class="special">,</span> <span class="identifier">std</span><span
                class="special">::</span><span class="identifier">move</span><span class="special">(</span><span
                class="identifier">self</span><span class="special">));</span>
      <span class="keyword">break</span><span class="special">;</span>
    <span class="keyword">case</span> <span class="identifier">reading</span><span class="special">:</span>
      <span class="keyword">if</span> <span class="special">(</span><span class="identifier">error</span><span
                class="special">)</span>
      <span class="special">{</span>
        <span class="identifier">self</span><span class="special">.</span><span class="identifier">complete</span><span
                class="special">(</span><span class="identifier">error</span><span class="special">,</span> <span
                class="number">0</span><span class="special">);</span>
      <span class="special">}</span>
      <span class="keyword">else</span>
      <span class="special">{</span>
        <span class="identifier">state_</span> <span class="special">=</span> <span
                class="identifier">writing</span><span class="special">;</span>
        <span class="identifier">asio</span><span class="special">::</span><span
                class="identifier">async_write</span><span class="special">(</span><span
                class="identifier">socket_</span><span class="special">,</span> <span
                class="identifier">buffer_</span><span class="special">,</span>
            <span class="identifier">asio</span><span class="special">::</span><span
                class="identifier">transfer_exactly</span><span class="special">(</span><span
                class="identifier">n</span><span class="special">),</span>
            <span class="identifier">std</span><span class="special">::</span><span class="identifier">move</span><span
                class="special">(</span><span class="identifier">self</span><span class="special">));</span>
      <span class="special">}</span>
      <span class="keyword">break</span><span class="special">;</span>
    <span class="keyword">case</span> <span class="identifier">writing</span><span class="special">:</span>
      <span class="identifier">self</span><span class="special">.</span><span class="identifier">complete</span><span
                class="special">(</span><span class="identifier">error</span><span class="special">,</span> <span
                class="identifier">n</span><span class="special">);</span>
      <span class="keyword">break</span><span class="special">;</span>
    <span class="special">}</span>
  <span class="special">}</span>
<span class="special">};</span>

<span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span
                class="identifier">CompletionToken</span><span class="special">&gt;</span>
<span class="keyword">auto</span> <span class="identifier">async_echo</span><span class="special">(</span><span
                class="identifier">tcp</span><span class="special">::</span><span class="identifier">socket</span><span
                class="special">&amp;</span> <span class="identifier">socket</span><span class="special">,</span>
    <span class="identifier">asio</span><span class="special">::</span><span
                class="identifier">mutable_buffer</span> <span class="identifier">buffer</span><span
                class="special">,</span>
    <span class="identifier">CompletionToken</span><span class="special">&amp;&amp;</span> <span class="identifier">token</span><span
                class="special">)</span> <span class="special">-&gt;</span>
  <span class="keyword">typename</span> <span class="identifier">asio</span><span class="special">::</span><span
                class="identifier">async_result</span><span class="special">&lt;</span>
    <span class="keyword">typename</span> <span class="identifier">std</span><span class="special">::</span><span
                class="identifier">decay</span><span class="special">&lt;</span><span
                class="identifier">CompletionToken</span><span class="special">&gt;::</span><span class="identifier">type</span><span
                class="special">,</span>
      <span class="keyword">void</span><span class="special">(</span><span class="identifier">asio</span><span
                class="special">::</span><span class="identifier">error_code</span><span class="special">,</span> <span
                class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span><span
                class="special">)&gt;::</span><span class="identifier">return_type</span>
<span class="special">{</span>
  <span class="keyword">return</span> <span class="identifier">asio</span><span class="special">::</span><span
                class="identifier">async_compose</span><span class="special">&lt;</span><span class="identifier">CompletionToken</span><span
                class="special">,</span>
    <span class="keyword">void</span><span class="special">(</span><span class="identifier">asio</span><span
                class="special">::</span><span class="identifier">error_code</span><span class="special">,</span> <span
                class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span><span
                class="special">)&gt;(</span>
      <span class="identifier">async_echo_implementation</span><span class="special">{</span><span class="identifier">socket</span><span
                class="special">,</span> <span class="identifier">buffer</span><span class="special">,</span>
        <span class="identifier">async_echo_implementation</span><span class="special">::</span><span
                class="identifier">starting</span><span class="special">},</span>
      <span class="identifier">token</span><span class="special">,</span> <span class="identifier">socket</span><span
                class="special">);</span>
<span class="special">}</span>
</pre>
    <h5>
        <a name="asio.reference.async_compose.h2"></a>
        <span><a name="asio.reference.async_compose.requirements"></a></span><a class="link"
                                                                                href="async_compose.html#asio.reference.async_compose.requirements">Requirements</a>
    </h5>
    <p>
        <span class="emphasis"><em>Header: </em></span><code class="literal">asio/compose.hpp</code>
    </p>
    <p>
        <span class="emphasis"><em>Convenience header: </em></span><code class="literal">asio.hpp</code>
    </p>
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            <div class="copyright-footer">Copyright © 2003-2022 Christopher M. Kohlhoff<p>
                Distributed under the Boost Software License, Version 1.0. (See accompanying
                file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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